This invention relates to a picture signal encoding device which is a block matching type motion compensation encoding device and which is for encoding a picture input signal into a picture output signal.
The input signal is typically a video signal of a television signal and represents a succession of pictures in successive periods which recur at a predetermined period, such as a frame period, so that each of the successive periods is equal to the predetermined period. The output signal represents the pictures with a predetermined bit rate. Each of the pictures has a movable part movable between a current picture represented by the input signal during a current period of the successive periods and a previous picture represented by the input signal during a previous period which is one predetermined period prior among the successive periods to the current period.
Although referred to as a part, the movable part may extend to a whole area of each picture. The movable part need not be different between two consecutive pictures. It is possible to divide or partition each picture into a plurality of blocks, each consisting of, for example, seven picture elements (pels) in each of seven consecutive horizontal lines.
A block matching type motion compensation encoding device is disclosed in Japanese Patent Prepublication (Kokai) No. 158,784 of 1980 for an invention by Akira Hirano filed on May 28, 1979, in Japan under Patent Application No. 65,853 of 1979. Another block matching type motion compensation encoding device is described in the Japanese language in an article contributed by Toshio Koga and three others including the above-named Hirano and the present inventor under the title of "Kaigi Terebi Singo no Ugoki Hosyo Huremukan Hugoka (Motion-Compensated Interframe Coding for Video Conference)" to "Tusin Hosiki Kenkyukai Siryo(Communication System Study Group Pamphlet)", Cs81-87, pages 85 to 90, published July. 22, 1981, by the Institute of Electronics and Communication Engineers of Japan. Still another block machining type motion compensation encoding device is described in a paper submitted by T. Koga and four others to IEEE International Conference on Communications, Jun. 19-22, 1983, under the title of "A 1.5 Mb/s Interframe Codec with Motion Compensation" (numbered 83CH1874.7/83/0000.1161). The encoding devices of the Koga et al article and paper are substantially identical insofar as the instant invention is concerned. Each will be called a Koga et al encoding device.
In the manner which will later become clear, the Koga et al encoding device comprises a vector detector. Supplied with the picture input signal, the vector detector detects displacements as motion vectors in the movable part. Detecting the motion vectors, the vector detector produces a motion vector signal representative of the motion vectors.
Supplied with the picture input signal and a subtracter input signal which will presently be described, a subtracter calculates a signal difference between the picture input signal and the subtracter input signal to produce a difference signal representative of the difference. A forward quantizer quantizes the difference signal into a quantized signal. A backward quantizer dequantizes the quantized signal into a dequantized signal. An encoder encodes the motion vector signal and the quantized signal into the picture output signal.
Supplied with the dequantized signal and an adder input signal which will shortly be described, an adder produces a sum signal representative of a signal sum of the dequantized signal and the adder input signal. A motion compensation predictor subjects the motion vector signal and the sum signal on a block-by-block basis to block match between the current and the previous pictures. Carrying out the block match, the motion compensation predictor predicts the picture input signal with motion compensation and with error minimization to produce a predicted input signal which is referred to as an optimum prediction signal in the Koga et al paper and is used as the adder input signal. The sum signal serves as a local decoded signal into which the dequantized signal is decoded.
As a consequence, a combination of the adder and the motion compensation predictor serves as a decoding and motion compensating circuit for subjecting the motion vector signal and the dequantized signal to the block match to produce the predicted input signal. A predicted signal supply connection supplies the predicted input signal to the subtracter as the subtracter input signal. The subtracter therefore produces the difference signal as a prediction error signal representative of the difference as a prediction error.
It is now understood that a whole combination of the subtracter, the forward and the backward quantizers, the decoding and motion compensating circuit, and the predicted signal supply connection serves as a predictive quantizing arrangement for producing the predicted input signal with the motion vector signal used to carry out the block match with the motion compensation and with the error minimization in producing the prediction error signal representative of the signal difference between the picture input signal and the predictive input signal. Furthermore, the predictive quantizing arrangement quantizes the prediction error signal into the quantized signal.
More briefly, the predictive quantizing arrangement derives the prediction error signal from the picture input signal and the motion vector signal and quantizes the prediction error signal into the quantized signal.
With a compact size of about 48.times.45.times.45 cm, the Koga et al encoding device is excellently operable when the predetermined bit rate is 1,544 Mb/s. A considerable amount of encoded information, however, still remains in the picture output signal.
As a result, it is difficult with the Koga et al encoding device to select a slower bit rate of, for instance 64 bit/s, as the predetermined bit rate even when the picture output signal is subsampled. Furthermore, the picture output signal must use an appreciably large amount of the encoded information when the movable part has a wide area. In such an event, the forward quantizer must quantize the error signal with a coarse quantization step. Alternatively, encoding of the picture input signal must intermittently be suspended. This gives an objectionable picture quality to pictures reproduced from the picture output signal. Inasmuch as such a slow bit rate is a problem to be solved by this invention, the slower bit rate will herein be referred to afresh as the predetermined bit rate.